Sime toll roads are built with the idea that the tolls pay for the cost of the road and maintenance of the road. Texas has no income tax, how high is the gas tax, how are roads funded?
The thing about toll roads here in Texas that angers folks the most (myself included) is how they have managed to bend the rules to enrich the companies that build them, not to mention how much of a dog and pony show their internal business management is. At the risk of digressing, I'll limit my response to the former part of that statement in an attempt to address your question.
So the state laws used to be written in such a way that ensured any toll roads would only be toll roads for as long as necessary to pay for themselves, and once they were paid for, they would stop being a toll road. Those laws where rewritten somewhat so that now a toll road remains a toll road as long as ANY road under the building authority's supervision is still being paid for.
What that means is that even after a toll road has paid back the bonds used to build it, it continues to remain a toll road forever, with all tolls going to the pertinent tollway authority that "owns" it. That goes against the intent of the original state laws concerning this matter, but only where earned money is concerned. When it comes to money paid out for things like law enforcement along the toll way roads, the original laws were left alone, which by the way specify the state is responsible for.
So the tollway authority gets a loan to build a road and then charges tolls to pay for that road. Once the road is built, the state is responsible for, and pays all costs associated with, law enforcement along the road. After the loan is paid back, nothing changes - the tollway authority continues to collect the tolls and the state continues to pay for law enforcement.